Carburetor Essential Tuning And Setup Tips - Four-Barrel Fundamentals

For more than 50 years, the classic Holley 4150-style four-barrel has remained a constant presence in the automotive world, as it evolved into almost countless versions for OEM and aftermarket applications. But in this age of electronic fuel injection, however, fewer and fewer enthusiasts know their way around the venturis, jets, and power valves of a traditional carburetor.

To put it simply, there's almost no reason to tinker with a carburetor these days, as our daily drivers haven't used them for more than two decades. We just don't have the experience any longer. So, while many enthusiasts relish the idea of building and working on a vintage engine, the perception for many is setting up and tuning a carburetor is a complicated black art-and it's enough to turn off those of us with limited experience.

That shouldn't be the case. While it's true that the variety of adjustable components on a typical four-barrel carburetor allow for an almost infinite number of tuning scenarios, there are a few basic steps to help select, set up, and tune your carb with surprising precision. In fact, even if you've never turned a screw on a Holley four-barrel, you should feel confident about installing one and getting your engine to run and idle very close to an optimal tune.

For this story, the tips are based on applications using the classic Holley 4150/4160-type vacuum-secondary, four-barrel (the non-Dominator-type), but most are applicable to other popular carburetors, be they of the single- or two-barrel variety, as well. With the following dozen tips to guide your way, you'll have your new carburetor breathing right and giving your muscle car the crisp, immediate throttle response that just doesn't come with electronic fuel injection.

1. Don't Super-Size It

When selecting a carburetor for your engine, don't overdo it. Generally speaking, a mild to moderate street engine doesn't need more than a 650- or 750-cfm carburetor. To zero in on the most appropriate carb size, multiply the cubic-inch displacement and maximum rpm and divide the product by 3,456. For example: a 350 engine multiplied by a 5,500-rpm redline and divided by 3,456 equals 557, or at least 557 cfm. In that case, a 650-cfm carb is sufficient. It's OK to go a little larger than necessary, but don't choose a carb that's rated at less than the minimum requirement.

2. Timing Is Everything

If the first couple of starts don't produce a quick firing and idle-even if they're not perfect-don't jump to the conclusion that it's a carb-tuning problem. The hard-starting scenario mentioned at the lead of this story could very well be due to improper ignition timing. So, before cursing the carburetor as you're ripping it off the intake manifold, double-check the ignition timing. Even an inaccuracy of only a few degrees could produce a hard-starting/no-idle condition.

3. Vacuum Secondaries vs. The Double-Pumper

Assuming you've got spot-on tuning, a carb with vacuum secondaries is easier to live with on a street car or one that sees only limited duty on the dragstrip. That's because the secondaries kick in automatically for more consistent performance; and they usually deliver better fuel economy. Vacuum secondaries are also tailored for the typical street-engine combination, such as a dual-plane intake and a "smaller" camshaft, as well as an automatic transmission-all of which typically provide better vacuum signals.

A Double Pumper carb with mechanical secondaries is controlled strictly by your right foot and uses an accelerator pump to shove more fuel down its throats when you quickly go to WOT. It uses more gas this way, but is more accommodating to big cams, single-plane intakes, and other vacuum- limiting factors. These qualities generally make the Double Pumper better suited to high-rpm combinations used mostly at the track.

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The power valve (known as a metering rod on a Carter-style carburetor) is a vacuum-activated feature that’s used to enrich the fuel mixture to prevent detonation or stumbling. They’re available in different sizes to match the flow requirements of the engine. On manual-transmission applications, a standard 6.5-inch power valve is adequate if the vacuum reading at idle is above 12 inches. For automatic applications, if the idle vacuum reading is below 12 inches, divide the number in half to determine the correct power valve size. For example: a 9-inch vacuum reading requires a 4.5-inch power valve.

5. Installing The Electric Choke

A key-on hot lead wire is connected to the positive (+) spade on the choke cap; and the negative (-) spade on the choke cap is wired back to the carb as a ground. It sounds simple enough, but many installers get it wrong. Don’t be one of them.

6. Electric Choke Fast Idle Adjustment

With the engine off, hold the throttle wide open; this drops a lever with a ¼-inch screw in it beneath the choke housing. To slow the idle, turn the screw counter-clockwise. Turn it clockwise to increase the idle speed.

7. Basic Fuel Level Adjustment

In the fuel bowl, a fuel level that is too low can cause stumbling when the throttle is opened quickly. Follow these simple steps to adjust it:

Remove the sight plug on the side of the fuel bowl

With the engine running, loosen the lock screw on top of the fuel bowl

Turn the nut on the fuel bowl clockwise to lower the fuel level and counter clockwise to raise the fuel level

8. Mixture Screw Setup

Getting the mixture screw adjusted properly is a must for preventing lean or rich conditions. Follow these steps:

Bring the engine to normal operating temperature and turn it off

Turn the mixture screws all the way in, and then back them out three full turns

Restart the engine

With engine idling at temperature, turn one screw in a quarter-turn and the next screw in a quarter-turn, repeating the process in quarter-turn increments until the engine rpm drops

When the rpm drops, turn the screws back out 1⁄8-turn

9. Selecting The Right-Size Jets

Whether you call them shooters or jets, installing the correct-size components is essential for smooth operation and stumble-free acceleration. Because every application is different, settling on the correct shooter size (orifice diameter) often comes down to trial and error tuning. Stumbling at take-off without black smoke from the tailpipe means the shooters/jets are too small. Stumbling with black smoke from the tailpipe means the shooters/jets are too large. Crisp, stumble-free acceleration and optimal vacuum mean the shooters/jets are just right.

10. Troubleshooting Bogging And Hesitation

Bogging or hesitation is annoying and gives the impression of low horsepower. It occurs when the secondaries open or “come in” too soon; and it can be corrected with a heavier secondary spring. Conversely, sluggish performance, especially at wide-open throttle, may be due to a secondary spring that is too heavy. Swap in a lighter one.

11. Overcoming Stumbling

Stumbling is a common malady experienced with a new or rebuilt carb that makes one wish for electronic fuel injection. But it’s pretty easy to diagnose and cure.

A stumbling condition at take-off is usually due to an inadequate accelerator pump fuel shot—assuming the shooter/jet size is correct (see Step 9). Inspect the pump shot with the engine off. Look into the carb and move the throttle; fuel should spray the instant the throttle is moved. If it doesn’t, turn the nut of the pump arm counter-clockwise one full turn and check the spray again. Continue until the spray is immediate with the throttle. If the pump spray is immediate upon inspection, check the following:

Inspect the pump diaphragm for a hole

Check the pump passage for debris or other impediments

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If you’re using an old carburetor that has sat unused for a long time—such as the time it took to remove, rebuild, and reinstall the engine—it’s pretty common for the gaskets in the metering block to dry out and/or shrink, causing idle problems. If everything else seems to check out, a rebuild of the carb to replace the old gaskets will likely do the trick.

One More Thing: Overdrive Transmission Compatibility

When using a Holley vacuum-secondary carb on a vehicle with a non-electronic overdrive automatic transmission, such as the AOD, a throttle kick-down bracket must be used. Otherwise, the transmission’s throttle valve (TV) cable won’t be positioned properly. When that happens, the transmission will shift too early or too late and the line pressures won’t be correct, eventually (and probably quickly) causing catastrophic (and expensive) transmission problems. The brackets are available from a variety of aftermarket vendors, such as TCI and B&M.

Tips For Setting Up Dual-Quads

If you’re replicating a factory system or going after the style of a period dual-four-barrel induction system, you’re right in thinking it’s trickier to set up than a simple single-carb system—but not that tricky. It’s just a matter of matching your tuning inputs consistently between the carbs. Follow these tips and your dual-quad muscle car should be running crisp and clean, with great throttle response. By the way, we’re starting with the assumption that the carburetors are installed on the intake manifold and have been set up enough to be in running condition.

Disconnect the throttle linkage between the carburetors.

Turn the idle speed screw on each carb all the way out and back in, then 1-1/2 turns from dead-closed. Repeat this step a couple of times as necessary to make sure the screws are in the same position on each carb.

Mixture adjustment--the vacuum method is the most accurate and requires connecting a vacuum gauge to at least one of the carburetors; it doesn't matter which one, but if you can hook vacuum gauges to both carbs, that's all the better for a more accurate overall reading. If the gauge is connected to the correct vacuum fitting, the gauge will register immediately upon engine start up. With the engine running, adjust the mixture screws--a quarter-turn at a time--to get the highest smooth-idle vacuum reading. Make the adjustments on one carburetor at a time rather than making matching adjustments simultaneously between them.

For properly balanced mixture settings on the carbs, the mixture screws shouldn't be more than 1/8 or 1/4-turn apart from one another. If they are, start over and make finer adjustments to balance the screw positions, but always keep in mind the goal is achieving a smooth idle.

Next, adjust the idle speed screws on both carburetors equally to achieve the desired idle speed. Check the idle in gear and out--there shouldn't be more than about a 250- to 300-rpm difference for most engines with "small" or mild cams--and no more than a 400-rpm difference for engines with large cams. If the idle speed difference is greater than that, the carbs may not have been adjusted properly. It could also mean the base timing is off and maybe even that the torque converter is too small, but start with the carb settings.

Connect the throttle linkage and adjust so that it fits snugly between the carbs, but not so tight that it holds one of them open.

Make sure both carbs open fully when the gas pedal is pushed to engage wide-open throttle.

For a progressive linkage, adjust the linkage so that the rear carb opens slightly before the front during light throttle application. Typically, this means a slightly looser linkage connection at the front carb, but one that still ensures both carbs go full-open at wide-open throttle. Importantly, a progressive linkage should only be used if the intake manifold has a common plenum, and not if each carb feeds only half of the cylinders directly. Otherwise, a progressive linkage could lead to an engine-damaging lean condition.

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Henry Ford And Holley Carburetors

It was none other than Henry Ford-at the dawn of the automotive age in the in early 1900s-who encouraged the Pennsylvania-based brothers George and Earl Holley to get into the carburetor business. The two had been tinkerers and built their own automobile, but as Ford's business began to grow, the brothers wisely followed his advice and established their landmark fuel systems company in 1903 and relocated to the Detroit area.

As the auto industry grew, so did Holley. The company's carburetors mixed air and fuel for just about everything on four wheels. Of course, their iconic four-barrel carburetor was a mainstay of the muscle car era and remains a core performance component more than four decades later.

The classic Holley 4150-series four-barrel made its first appearance on the '57 Ford Thunderbird. Its use in Detroit's factory supercars mushroomed and it became a popular aftermarket product. The company was sold to Ohio-based Colt Industries in 1968 (the same year the larger, "Dominator"-series carb arrived, designed for NASCAR racing), remaining part of that company until its sale to a private equity firm in 1998. New cars were fuel-injected, so manufacturers' needs for carburetors dried up. Holley necessarily shifted its focus to aftermarket performance, but that was tough, too, as enthusiasts embraced fuel injection. In 2008, Holley filed for bankruptcy and emerged from it in 2010.

Regardless of the ups and downs of the past century, it was the urging of ol' Henry himself that helped make Holley a performance icon.